A polymer electrolyte fuel cell (hereinafter referred to as “PEFC”) causes a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas, such as air, to electrochemically react with each other to generate electric power and heat at the same time. The hydrogen-containing fuel gas is obtained by reforming a material gas, such as a city gas. At this time, a reaction shown by Chemical Formula 1 occurs in an anode, and a reaction shown by Chemical Formula 2 occurs in a cathode.H2→2H++2e−  (Chemical Formula 1)½O2+2H++2e−1→H2O  (Chemical Formula 2)
During the electric power generation of the PEFC, a part of water generated in the cathode back-diffuses and moves to the anode.
A general structure of a conventional unit cell (cell) in such PEFC is shown in FIG. 25.
As shown in FIG. 25, a cell 80 of the PEFC includes a membrane-electrode assembly 75, gaskets 76, and electrically conductive separators 77. The membrane-electrode assembly 75 is configured such that an electrode 74 formed by a catalyst layer 72 and a diffusion layer 73 is disposed on each of main surfaces of a polymer electrolyte membrane 71. In the cell 80, when viewed from a thickness direction of the polymer electrolyte membrane 71, a gap is formed at an end portion of a region between the polymer electrolyte membrane 71 and the diffusion layer 73 at which portion the catalyst layer 72 is not provided. At this gap, nothing supports the polymer electrolyte membrane 71. Therefore, if the polymer electrolyte membrane 71 is thin, the following problems may occur.
For example, in a case where the electrode 74 and the polymer electrolyte membrane 71 are bonded to each other by, for example, hot pressing, an end portion of the diffusion layer 73 of the electrode 74 may contact the main surface of the polymer electrolyte membrane 71, and this may damage the polymer electrolyte membrane 71. Moreover, in the case of fastening the cell 80, mechanical stress may be applied to the polymer electrolyte membrane 71, and this may damage the polymer electrolyte membrane 71. Further, the polymer electrolyte membrane 71 may tear by a pressure difference between the fuel gas and the oxidizing gas. If the polymer electrolyte membrane 71 is damaged, serious safety problem may occur, for example, the fuel gas and the oxidizing gas may burn by cross leakage of these gases.
To solve the above problems, known is a seal structure of a solid polymer electrolyte fuel cell in which a frame-shaped protective membrane is attached to a polymer electrolyte membrane (see Patent Document 1 for example).
FIG. 26 is a schematic diagram schematically showing the seal structure of the solid polymer electrolyte fuel cell disclosed in Patent Document 1.
As shown in FIG. 26, a frame-shaped protective membrane 220 formed by a fluorocarbon resin-based sheet is disposed on a main surface of a solid polymer electrolyte membrane 210 such that an inner peripheral portion thereof is covered with an electrode 213. Moreover, a gas sealing material 212 is disposed to surround the electrode 213 such that a gap 214 is formed between the gas sealing material 212 and the electrode 213. With this, the protective membrane 220 is sandwiched between the gas sealing material 212 and the solid polymer electrolyte membrane 210 and between the electrode 213 and the solid polymer electrolyte membrane 210, and the protective membrane 220 reinforces the solid polymer electrolyte membrane 210 at the gap 214. Therefore, the solid polymer electrolyte membrane 210 can be prevented from being damaged without increasing the thickness of the solid polymer electrolyte membrane 210.
Patent Document 1: Japanese Laid-Open Patent Application Publication Hei 5-21077